The invention relates to monitoring brain function by measuring and processing time-varying spontaneous electrical potentials which exist between different areas of a patient's scalp and/or body, and/or time-varying evoked potentials produced in response to stimuli delivered to the patient.
More specific aspects of the invention relate to monitoring the occurrence and ascribed medical significance of changes in a patient undergoing a medical procedure relative to an earlier state of the patient, at a time selected for medical relevance to the planned procedure, and to producing, in case of statistically and medically significant deviations from the self-norm, alarms which convey information both as to the occurrence of such deviations and so to the ascribed medical significance of the deviation itself and its persistence. One non-limiting example is monitoring relevant brain functions of a patient undergoing surgery which has the potential of impairing the regional blood supply to the cortex, and producing suitable alarms in case of detecting medically significant blood (and oxygen) supply impairment. Numerous other examples are identified below.
It is well known that spontaneous electrical potentials (SP) exist between different areas of a patient's scalp and/or other body parts, and that a record thereof over a period of time, called an electroencephalogram, or EEG for short, can be studied in an effort to relate the SP to brain activity and function. It is also known that when a patient is subjected to stimuli, evoked potentials (EP) tend to be superimposed on the normally present EEG potentials, and that the EP waveforms can also be studied in an effort to relate them to brain activity and function. It is further well known that the SP and EP measurements produce extremely weak signals which are subject to many adverse influences and are therefore difficult to measure accurately and precisely, and that the medically significant information contained in them is typically severely obscured.
Traditionally, a clinician evaluating SP and EG measurements relies on visual inspection of raw or averaged waveforms in seeking to extract the relevant diagnostic features. Aside from depending to a great extent on the experience, skill and judgment of the particular clinician, this method is immensely complicated by the high inherent variability of the relevant waveforms. In an effort to enhance the available information, various machine-processing techniques have been applied to the raw measurements which, in addition to seeking to enhance the signal-to-noise ratio, match the information derived from a particular patient with a norm derived from a large population of persons who are believed to be "normal" with respect to the brain function of interest. An example of such measurements and processing is disclosed in U.S. Pat. No. 4,201,224 (hereby incorporated by reference in this application) and in the prior art discussed in it. Further background information can be found in U.S. Pat. Nos. 4,216,781; 4,279,258 and 4,188,956. In addition, machine-implemented techniques have been applied in seeking to find whether a significant difference exists between two sets of measurements taken from the same patient, such as in correlating the outputs of two electrodes placed in bilateral symmetry on the patient's head (e.g., U.S. Pat. No. 3,696,808), and carrying out a t-test to determine whether a statistically meaningful difference exists between the brainwaves generated in response to two different sets of stimuli (e.g., U.S. Pat. No. 3,901,215).
While in many cases it is important to know if a statistically and medically significant difference exists between the electrically measured brain function of a patient and the similarly measured brain function of a statistically and medically significant "normal" population, it has been discovered in the course of making this invention that in certain medical procedures it is additionally, or instead, advantageous to know if during the procedure certain changes have taken place in brain functions related to the particular procedure, as compared to the same functions of the same patient at a certain time before the procedure. As a nonlimiting example, in the case of a patient admitted for surgery in the course of which the regional blood (and hence oxygen) supply to the cortex, can be impaired, it has been discovered that it is advantageous and possible through this invention, to know not only the difference between the brain function associated with oxygen supply to the cortex as between this patient and a normal population but also changes in the function of the same patient as between a stable state with respect to this particular surgery (e.g. upon admission or upon administering anesthesia) and times during the surgery selected such that ample warning of a significantly reduced regional oxygen supply would be received before significant damage due to oxygen starvation can occur.
In accordance with a particular and nonlimiting example of the invention, brain functions of a patient which are selected for medical relevance to a planned medical procedure, are electrically measured at a time at which the patient is in a state selected for medical relevance to the procedure. In the nonlimiting example referred to immediately above, the medical procedure is the surgery, the state selected for medical relevance to the procedure is a state in which the patient is judged to be medically stable with respect to regional blood supply to the brain, e.g. soon after admission or soon after administration of anesthesia, and the brain functions selected for medical relevance to the procedure can be, e.g., the slow wave brain activity.
The measurements are processed to produce a self-norm comprising, for each brain function, a respective statistically and medically significant mean measurement and a respective statistically and medically significant standard deviation or variance measurement. In the same example, a sufficient number of measurements should be taken, in view of the anticipated variability of slow wave activity, so as to be able to derive a mean measurement which is not only statistically significant (as determined on the basis of known statistical criteria) but is also medically significant with respect to the known or perceived relationship between slow wave activity and regional oxygen supply to the cortex. Furthermore a sufficient number of measurements should be taken to allow not only the derivation of the desired mean but also the derivation of a similarly statistically and medically significant variance (or, in the simplified case, standard deviation).
After the procedure begins, the same brain functions of the same patient are electrically measured at each of a sequence of time intervals selected for medical relevance to the procedure. In the same nonlimiting example, the slow wave brain activity is measured during surgery at time intervals which are less than the duration of regional cortex oxygen starvation which could cause significant damage. For example, on the belief that significant damage occurs after 15-30 seconds of oxygen starvation, new measurements can be taken every five seconds.
Each new set of measurements is tested for statistically and medically significant change from the self-norm. In the nonlimiting example discussed here, each new set of measurements comprises slow wave measurements taken under measurement conditions sufficient to give a statistically and medically significant sample, and the desired test comprises testing the new set against the self-norm in a manner taking into account the statistically and medically significant mean and variance of the self-norm.
If such a change is detected, an indication is produced which conveys information not only as to the occurrence of the change but also as to the ascribed medical significance of the change itself and its persistence. In the example discussed here, the first indication of a change can result in a visual alert, a consecutive indication (i.e., a change persisting over two measurement intervals) can lead to an auditory alert for a possible emergency, a third consecutive indication can lead to an auditory and/or visual announcement of a possible emergency, and a fourth consecutive indication can lead to automatically initiated emergency procedures. If a change persists through two or three measurement intervals, but no change from the self-norm is found in the next interval, the system can go to the next lower level of alert.
In the case where two or more different brain functions are used in the self-norm and in the periodic measurements during the procedure, the change in each function from the corresponding component of the self-norm can be converted, in accordance with the invention, to a dimensionless probability of significant deviation, and these probabilities with respect to two or more brain functions can be combined, e.g. through vector addition of weighted or unweighted individual probabilities, into a single indication, e.g. in vector form, of a statistically and medically significant change from the self-norm.
As nonlimiting examples, the invention can be applied to monitoring a patient's brain functions against a self-norm in the case of surgical procedures which have the potential of affecting brainstem functions, or spinal cord functions, or thalamic cortical functions, surgical procedures which have the potential of affecting the EEG brain activity, surgical procedures which have the potential of affecting the ability to detect and process sensory stimuli, and medical procedures which comprise drug or other treatments having the potential of affecting electrically measurable brain function.